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Last Thursday, we discussed news that millionaire Dennis Tito was planning a private mission to Mars in 2018, but details were sparse. Now, reader RocketAcademy writes that Tito has provided more information about the tip, and that he intends the mission to be manned:
"Dennis Tito, the first citizen space explorer to visit the International Space Station, has created the Inspiration Mars Foundation to raise funds for an even more dramatic mission: a human flyby of the planet Mars. Tito, a former JPL rocket scientist who later founded the investment firm Wilshire Associates, proposes to send two Americans — a man and a woman — on a 501-day roundtrip mission which would launch on January 5, 2018. Technical details of the mission can be found in a feasibility analysis (PDF), which Tito is scheduled to present at the IEEE Aerospace Conference in March. Former NASA flight surgeon Dr. Jonathon Clark, who is developing innovative ways of dealing with radiation exposure during the mission, called the flight 'an Apollo 8 moment for the next generation.'"

Even if the mission goes 100% to plan, the cancer risk alone is probably a death sentence for the two passengers.

It's right there in the article:

The expected total radiation exposure is below NASA’s accepted lifetime limit for a middle-aged crew, Dr. Clark said. Clark expects that radiation exposure would result in a 3% excess cancer risk over the crew’s lifetime.

You may dispute the numbers (but I don't see how you could, given that the details of the spacecraft aren't known), but I think many people would be willing to take that risk - smokers probably face worse cancer odds than that.

One concern is the life of their pressure suits. Lunar fines are very abrasive and Apollo surface suits had a short working life. Martian fines may cause similar problems.

I don't think the fine particles on Mars will for the most part resemble those on the Moon. Mars has had wind blowing the particles around for a very long time, smoothing out the rough corners on the particles. The Moon clearly has no wind. The particles on the Moon likely formed via meteorite impact ejecta, either from shattered rock or by condensation from vaporized rock. After formation, there would likely be less corner erosion of fine particles due to the lack of wind. Thus the Moon's fine particles are quite abrasive.

They're in very risky territory. Outside the earth's magnetosphere, things are much much worse than LEO.(where pretty much every astronaut except the Apollo ones spend all their time.)

There is a good paper on it here:http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070010704_2007005310.pdf

They estimate a dose of 1.03 Sv for a 600 d mars flyby, which would be just over the lifetime limit for most space agencies.

Also:http://en.wikipedia.org/wiki/Health_threat_from_cosmic_rays

5% brain death seems like a pretty serious problem....

I'd add that these are conservative estimates for dose. An unlucky event like a solar flare could make things much worse. A solar flare is sometimes predictable, so astronauts might be able to hide behind extra shielding(like their water supply). This would prevent acute radiation poisoning, but still substantially increase lifetime dose.

There is very little to learn. We already had ground simulations of the flight, and they were generally unsatisfactory. Humans cannot sit in a tin can for two years and retain sanity. That alone overwhelms all the other issues, of which there are many

Russia, the EU and China conducted a joint simulation with mission lengths of 15, 105 and 520 day durations. After the 520 day mission:

The 520-day final stage of the experiment, which was intended to simulate a full-length manned mission, began in 3 June 2010 and ended on 4 November 2011.[8][9][10] This stage was conducted by a six-man international crew, consisting of three Russians, a Frenchman, an Italian/ Colombian and a Chinese citizen.[10] The stage included a simulation of a manned Mars landing, with three simulated Mars-walks carried out on 14, 18 and 22February 2011.[11][12] The experiment ended on 4 November 2011, with all the participants reportedly in optimal physical and psychological condition.[10]

In January 2013, the Proceedings of the National Academy of Sciences reported that four of the six crew members had considerable problems sleeping, and some avoided exercise and would hide away from the others, in behaviour compared to animal hibernation.[13]

Insomnia and exercise avoidance doesn't sound all that unsatisfactory. Though I don't think it's possible to truly simulate a mission to Mars here on Earth when the participants know that if things go very bad, they are just an escape hatch away from help. I think the only way to do a true simulation would be if the participants really thought that they were in a space capsule, which is pretty hard to do when gravity gives it away.

They had 6000 volunteers for the long mission - I suspect that an actual mission to mars will result in many more volunteers, despite the risks.

On the contrary. There will be a lot of science done. It will be in the realm of long duration space travel and its affects on humans, rather than on gathering data on Mars, but there is plenty of opportunity for science on such a mission.